The present invention relates to a position control circuit for positioning a magnetic read-write head on a selected track of a magnetic disk in a magnetic disk device using a servo system for speed control and position control.
In a magnetic disk device, a magnetic head is positioned on a selected track by use of a servo system which performs a seek control. The head is then shifted to the selected track at high speed by performing speed control. Finally, the head is accurately positioned on the selected track by performing position control. (In multi-disk devices, tracks on disks with the same number of tracks are referred to as cylinders. The present invention does not depend on the number of disks and thus the term track is used in the following description.) A position control circuit includes a differentiating circuit, an integrating circuit and a proportional circuit for performing the position control by a proportional plus integral plus differential action, i.e., a PID action.
FIG. 1 illustrates a position control circuit of a servo system for performing a seek control in a prior art magnetic disk device.
In FIG. 1, a servo circuit 10 and a servo-controlled system 20 are shown. The servo circuit 10 has a position control circuit 11. The other circuits are omitted. In the servo-controlled system 20, a magnetic read-write head 21 is moved on a disk 22 by a voice coil motor (hereinafter referred to as VCM) 23 to write on and read from the disk 22.
The position control circuit 11 has an integrating circuit 110, a differentiating circuit 120, a proportional circuit 130 and adders 140 and 150.
In this arrangement, the servo circuit 10 shifts the head 21 toward a selected track at high speed by means of speed control and then the control is switched from speed control to position control so that the head 21 is positioned on the selected track.
For the position control, the adder 140 produces a position signal Ps indicating the current position of the head 21 and an error signal .DELTA.P indicating the difference between the current position of the head 21 and the selected track position Po.
In the error signal .DELTA.P, the integrating circuit 110, the differentiating circuit 120 and the proportional circuit 130 perform the integral action, the differential action and the proportional (amplifying) action, respectively. Output signals of the integrating circuit 110, differentiating circuit 120 and proportional circuit 130 are added together to produce a position error signal .DELTA.P.sub.E as a result of the proportional plus integral plus differential action (PID action). The position error signal .DELTA.P.sub.E is amplified up to a predetermined power level and then applied to a VCM 23 of the servo-controlled system 20 to drive (move) the head 21. That is, the position control is performed by a position control loop comprised of the head 21, the position control circuit 11, the VCM 23 and the head 21, so that the head 21 is positioned on the selected track.
With recent advances in the high-speed version of track seek, a one-track seek is now considered, which utilizes the PID control by the position control circuit 11.
FIGS. 2A(a)-2A(b) illustrate normal operating waveforms in the position control for one-track seek. More specifically, FIG. 2A(a) illustrates an operating waveform of the position signal Ps and FIG. 2A(b) illustrates an output waveform of the integrating circuit 110. With external force applied to the head 21, the position signal Ps produces a transient oscillation as shown which rapidly converges on 0 by means of the PID action of the position control circuit 11. In that case the output (integrated signal IS) of the integrating circuit 110 will have a fixed level which corresponds to the position of the selected track.
However, when the external force applied to the servo-controlled system (head) varies and thus the head position signal Ps varies as shown in FIG. 9B(a), the variations are added in the integrator 110 so that the resulting integrated signal IS varies as shown in FIG. 9B(b). The output of the integrator 110 of the position control circuit 11 in the position control is mostly an integrated value of or external force applied to the head in an on-track state. The variation in the integrated signal IS causes the head position signal Ps to vary and the integrated signal IS is also further varied. It thus takes a long time for the position signal Ps and integrated signal IS to become stabilized, i.e., for a seek to be terminated.
The variation in external force refers to the variation in mechanical external force applied to the head 21. If the movement of the head 21, namely, the seek operation, is performed slowly, the external force will vary little. With a fast seek operation, the external force will vary transiently.
Although to achieve a high-speed seek, it is desirable for a track seek to be made while the PID control is performed the position control circuit of the prior art servo circuit has a problem that it is subject to the influence of variations in the external force because of the presence of the integrator. Hence, the settling of the head becomes unstable, thereby hindering stable fast track seek.